This invention is in the field of recombinant Plasmodium falciparum polypeptides and vaccines.
The major merozoite surface protein of Plasmodium species has been shown to be a target of varying degrees of protective immunity against the asexual blood stages in rodent and human malarias.
For example, vaccination of mice with purified P230, the major merozoite surface protein of the rodent malaria Plasmodium yoelii, has resulted in reduced parasitemias in comparison to controls upon intravenous challenge with a lethal dose of parasitized erythrocytes (1). Mice have also been protected against P. yoelii by passive transfer of a monoclonal antibody specific for P230 (2) and against (rodent malaria) Plasmodium chabaudi adami challenge by passive immunization with a monoclonal antibody specific for the homologous 250-kDa molecule of this Plasmodium species (3). The ability to confer resistance to parasite challenge by passive transfer of antibodies suggests that antibody-mediated mechanisms play an important role in antigen-specific immunity to malaria.
Despite these findings, however, no commercially viable vaccine has been developed against the major merozoite surface antigen of the major human malaria pathogen, Plasmodium falciparum. 
For example, using naturally derived materials, such as the precursor of the major merozoite surface protein alone (gp195: 195-200,000 Da molecular species), gp195 mixed with certain of its natural processing fragments, or a natural processing fragment by a itself partial protection against Plasmodium falciparum infection has been achieved by some researchers (4,5,6). An effective vaccine against Plasmodium falciparum should not convey merely partial protection, i.e. a partial lowering of parasitemia, since even low parasite mias of this organism cause serious illness. A commercially useful vaccine should substantially eliminate parasitemia.
In one instance, substantially complete protection using natural materials against Plasmodium falciparum challenge has been achieved in Aotus monkeys, in particular by the use of a mixture of gp195 and some of its natural processing fragments obtained by affinity purification using a monoclonal antibody, designated Mab 5.2 (7). In follow on experiments using Mab 5.2 affinity purified, parasite gp195, a correlation was found between protection against infection with Plasmodium falciparum and the ability of serum antibodies to strongly inhibit parasite growth in vitro. In particular, monkeys and rabbits hyperimmunized with Mab 5.2 affinity purified parasite gp195 in complete Freund""s adjuvant produced antibodies that inhibited in vitro parasite growth (8).
The difficulty in developing an effective naturally derived vaccine, however, has been compounded with the difficulty in developing an effective recombinant or synthetic vaccine. Recombinant or synthetic vaccines are desirable for several reasons. They have the potential to focus immune response on the most effective portion of gp195, an important advantage since there may be decoy determinants in gp195 which prevent the most effective response. Also, more homogeneous preparations are possible using recombinant techniques than in preparations of naturally derived products. In addition, recombinant and synthetic based vaccines avoid the potential contamination of naturally derived gp195 with pathogens from its human source.
Although a number of investigators have designed and tested gp195-based synthetic peptides and recombinant products as vaccine antigens no strongly protective vaccine has resulted. Thus, synthetic peptides corresponding to various segments of the N-terminal 83 kDa processing fragment of gp195 induced antibodies in rabbits which displayed only a low level of crossreactivity with asexual blood stage parasites (9). One of these synthetic peptides, corresponding to a non-repetitive, conserved sequence, partially protected Saimiri monkeys against Plasmodium falciparum challenge (9). In a vaccination study in Aotus monkeys using an 83 kDa processing fragment-based recombinant polypeptide produced in E. coli there was no significant difference between the course of infection of control animals and animals immunized with the recombinant polypeptide (in addition, very low levels of antibodies cross-reactive with native gp195 by immunofluorescence were induced) (10). A bacterial recombinant polypeptide based on a fusion of two conserved regions located towards the amino terminus and center of the gp195 molecule induced only low IFA titers when used to immunize Aotus monkeys (11) and two out of five immunized animals were partially protected.
Holder et al. studied two recombinant polypeptides which corresponded to portions of the 42 kDa C-terminal processing fragment of gp195 (p42) fused to trp E and xcex2-galactosidase carrier sequences, respectively (12). While immunized animals produced high antibody titers against the carrier portion of the recombinant polypeptides, much lower titers were detected against the gp195 antigen. Some of the Aotus monkeys immunized with both of these recombinant polypeptides were partially protected against parasite challenge.
Murphy et al. have attempted to recombinantly produce portions of the p42 antigen of the Wellcome isolate of gp195 in insect host cells. (Gp195 is believed to exist in at least two allelic forms, of which the Wellcome isolate (xe2x80x9cWellcome allelexe2x80x9d) and the MAD isolate (xe2x80x9cMAD allelexe2x80x9d) are representative (13)). While Murphy et al. reported producing a product which folded in a similar manner to the natural antigen, they did not report obtaining a purified polypeptide (only multiply banded antigens speculated to have resulted from post-translational processing or degradation). No follow-up studies have been reported using any materials obtained.
A mixture of three synthetic peptides, one peptide from the 83 kDa processing fragment of gp195 and two non-gp195 malaria peptides, partially to completely protected monkeys against parasite challenge; a hybrid synthetic polymer including the sequences of the three synthetic peptides in addition to a circumsporozoite region was reported to provide a delay or suppression of parasitemias (14, 15, 16). Field trials of this hybrid are under way. It is unclear whether any gp1 95 epitopes in the mixture or hybrid resulted in any protection. In addition there have been two studies reported which were unable to duplicate the prior results obtained using the peptide mixture (17) or the hybrid peptide multimer (18).
Thus, there has been no gp195-based recombinant or synthetic vaccine antigen which has been shown sufficiently effective against Plasmodium falciparum challenge.
The present invention relates to a vaccine made with baculovirus produced p42 antigen (BVp42), or an effective immunogenic part thereof, which provides strong protection against Plasmodium falciparum infection. In addition, BVp42 is capable of inducing antibodies that are extensively crossreactive with different parasite strains. The invention also relates to a method of producing BVp42 based antigens without cleavage or degradation by infecting an insect host cell with a recombinant baculovirus vector that contains DNA encoding a p42 amino acid sequence that is characteristic of the MAD-allele. The DNA sequence is operably linked to a baculovirus polyhedron promoter.